The present invention relates to a perpendicular magnetic recording method and apparatus, more particularly to a method and apparatus for superimposing a plurality of signals with different frequencies on a perpendicular magnetic recording medium.
In what is known as a perpendicular magnetic recording technique, signals are recorded on a magnetic recording medium with an axis of easy magnetization in the direction perpendicular to the magnetic surface. This is accomplished by magnetizing the medium in the direction perpendicular to its magnetic surface. This technique has been attracting attention for its high density recording possibilities. This perpendicular magnetic recording technique may be applied to a variety of recording devices such as video apparatus, audio apparatus, memories for electronic computers, and so forth. Generally, with this technique, one signal will be recorded on one recording track.
With the more conventional longitudinal recording method, depending upon the purpose of its use, a plurality of signals with different frequencies may be superimposed on one recording track. For instance, in home use video cassette recorders, the low frequency conversion chrominance signal is recorded simultaneously supeimposed on the low frequency carrier FM luminance signal, the frequency of which is higher than that of the low frequency conversion chrominance, using the same recording head. It is important in a superimposed recording of this kind that the recording is made by supplying the magnetic head with a previously determined optimum recording current so as to be able to reproduce sufficiently strong outputs for each of the superimposed signals during reproducing. In particular, in a superimposed recording, an important problem is how to select the recording current of the high frequency signal for the high frequency bias.
Such a superimposed recording has so far been accomplished for the longitudinal magnetic recording method alone. In the superimposed recording of the longitudinal magnetic recording method, it has been known that the values of the reproduced outputs of the high and low frequency signals are maximized with approximately the same recording current for the high frequency signal. Therefore, in the longitudinal recording method, a satisfactory superimposed recording characteristic can be obtained by first selecting the recording current for the high frequency signal which gives rise to a maximum reproduced output of the high frequency signal, and then by determining the recording current for the low frequency signal so as to have a reproduced output level of the low frequency signal which is nearly equal to the reproduced output level of the high frequency signal.
However, by carrying out a superimposed recording for the perpendicular recording method by selecting the recording currents for the high frequency signal and the low frequency signal by a method similar to the longitudinal recording method, it was found that a satisfactory superimposed recording characteristic was not obtainable due to large distortion levels, the impossibility of obtaining a sufficiently large output level of the reproduced signal, and so forth.